What happens when I park my number?
By parking your number you link your regular phone number to a dedicated temporary service number in your home country.
Basically, "parking" refers to call forwarding (or call diverting as they say in UK), which is a service that redirects a telephone call to another destination where the desired called party is available. After you've parked your number, calls to your regular number get redirected to the service number and then to your foreign SIM, allowing you and your caller to avoid roaming charges.
You need to park your number before departure and in your home country, or you will not be able to use your regular number abroad. Use the "Set Reminder" feature to park your number on time.